An Investigation of Some of the Relationships between Copper, Iron, and Molybdenum in the Growth and Nutrition of Lettuce: II. Response Surfaces of Growth and Accumulations of Cu and Fe

A study was made to determine some of the interrelations of Cu, Fe, and Mo in the growth and nutrition of lettuce. The objectives of this study were: (1) to determine the importance of interactions among Cu, Mo, and Fe; (2) to determine if form of N or source of Fe affects interactions involving Cu; (3) to determine which concentrations of Cu, Mo, and Fe result in the greatest growth; and (4) to determine if levels of Fe and Mo, sources of Fe, or form of N affect the critical level of Cu in the plant. Solutions with varying concentrations of Cu, Fe, and Mo were imposed on four series of treatments involving combinations of ferrous iron, ferric iron, all nitrate nitrogen, and 1/4 ammonium plus 3/4 nitrate nitrogen. The experiment was conducted using solution culture in a rotatable design. This design permitted calculations of response surfaces from a much smaller number of treatments than would have been necessary with complete factorials. All of the variables affected growth and accumulation of ions. Illustrations are given for some of the relationships which appear to exist among Cu, Mo, Fe, sources of N, and sources of Fe. Pronounced interactions were obtained between Cu and Fe. The growth of lettuce at any one level of Cu was affected by Fe supply. The reverse relationship was also observed. The Cu content of leaves was affected by Fe supply as was Fe content by Cu supply. The interaction between Cu and Fe on Cu content was affected by source of N and source of Fe. The effects of Cu and Mo on Fe content and the interactions between Cu and Mo, Cu and Fe, and between Fe and Mo were much more pronounced in the series containing all NO₃ - nitrogen. Curves of yield vs. Cu content of tops suggested that there is not any single level of Cu in the plant which represents an optimum level. The Cu content corresponding to maximum yield appeared to be influenced by levels of Fe and Mo in solution, source of Fe, and source of N. In general, form of nitrogen had a more pronounced effect on growth and accumulation than did the valence of Fe. Implications of the relationship suggested by this study and the use of response surfaces as an aid in studying nutrient interactions are discussed.